One well established method of defrosting evaporators in refrigeration systems is by forcing hot gas from the compressor of the system through the evaporator so that the evaporator acts as a condenser. The heat given up by the evaporator under those conditions melts the ice and frost that has formed on the evaporator coils and fins during normal refrigeration operation. Typically the hot gas defrosting is accomplished by diverting valves in the refrigerant path which are switched to accomplish defrosting mode to divert the hot gas through the evaporator, either in the same direction as the refrigerant flow during refrigeration, or in the opposite direction. The defrosting can be accomplished on a time basis so that a defrost cycle is run at selected intervals, or it can be done on a demand basis through the use of frost sensors and the like for detecting frost buildup on the evaporator coils.
While hot gas defrosting has proved to be very useful and advantageous, there may be certain areas in which problems can occur in practice which can lead to faulty operation or less than optimum efficiency.
The present invention provides improved control over several areas of the hot gas defrosting process so as to provide efficient defrosting at a high speed and with little or no waste of energy on the defrosting process. Preferably the invention is used in conjunction with restricting the refrigeration system during a defrost cycle by switching the condenser and perhaps other components out of the refrigerant path. By thus restricting the system, all heat absorbed by the refrigerating evaporator or evaporators is used in the defrosting process. This helps to provide maximum speed of defrosting and maximum energy efficiency, so there is little or no waste heat. The control system of the present invention operates to provide the correct amount of refrigerant charge in the operating part of the system during the defrost cycle. This is important because if there is too little charge in the operating portion, there may not be enough refrigerant to maintain the refrigerating process, without which it is not possible to get a good defrost process for the defrosting evaporator or evaporators. This would mean a low energy efficiency and a corresponding waste of energy and money. If the charge in the operating portion during defrost is extremely low, there may not be any defrosting operation at all.
On the other hand, if there is too much refrigerant charge in the operating part of the system during a defrost cycle, this could lead to problems. The excess refrigerant would tend to collect as liquid in areas of the defrosting evaporator which are to be defrosted, and this liquid collection would prevent hot gas from flowing through those areas of the evaporator and would prevent defrosting. The result would be slow and incomplete defrosting. A further problem that can be aggravated by excess refrigerant charge in the defrosting loop is that after the completion of the defrost cycle, when the diverting valves, etc. are switched back to normal refrigeration mode, this may result in slug back of liquid refrigerant into the suction line which could cause damage to the compressor. In some prior art systems it has been necessary to introduce an accumulator in the suction line in order to trap such liquid refrigerant to prevent slug back at the end of defrost. This need is eliminated in the present invention, wherein the control system effectively determines where the liquid refrigerant will be within the system and controls the transition from defrost mode back to refrigeration mode in order to prevent the liquid refrigerant from entering the suction line.
Improvements in the design of the evaporators in terms of refrigerant path circuiting and valving control can advantageously be used with the above improvements in defrost cycle control, to achieve improved speed and uniformity in the defrosting of the evaporators. These features and advantages are pointed out in greater detail with reference to the description of the preferred embodiment herein.